No Mercy

The objective of a 2 scout uber is to use your uber advantage to pick the opposing medic without uber, (or demoman in some a case where he is more important eg badlands last?) The ubered players and medic will have to overextend and commit into enemy Territory to get the kill, so the rest of the teams job is to follow in cautiously and prepare to cover the medic post-uber, ideally catching any other enemies distracted by the uber (without taking large risks!).

The reason you choose 2 scouts over ubering any other class is because they are faster* and more able to chase down the escaping victims.
(*in theory.)

I cannot think of a situation where you have 6 players and uber advantage and this the above is actually true. With any space to move soldiers and demoman can propel themselves faster than scouts with jumps, and are very rarely less vulnerable to being bounced than a flat running scout. Nevermind the mitigating factors of no uber splash damage and the increased reliability on your scouts not choking (they MUST aim true).

If you have 6 players alive i cant think of a situation where it is optimal to employ a double scout uber. I would prefer a scout+soldier/demo in the above case, if even any scouts. this is not to say it is a BAD strategy, i just dont think it is ever the best choice.

It would be a good choice (to uber both scouts in) if you are in say a 4v4 with uber vs no medic on the tail end of a rollout. However this is less about a 2 scout uber strategy and more just "use uber and anyone alive and kill everything"